GROWING UP ON THEIR OWN: SMART PLASTICS ORGANIZE THEMSELVES
INTO USEFUL OPTICAL DEVICES

January 15, 1999

In one of the first examples of molecules building
themselves into useful synthetic microstructures with little
human intervention, chemical engineers have created plastic
materials that assemble themselves into sophisticated optical
devices known as photonic crystals. Samson Jenekhe and Linda Chen
of the University of Rochester describe their work on a process
known as "hierarchical self-assembly" in the Jan. 15 issue of
Science.

Just last year the same team created the largest synthetic
structures ever made by self-assembly, where molecules organize
themselves into discrete microscopic objects, such as hollow
spheres. In the current work they've gone one step further,
developing methods to make billions of those objects come
together to form even larger, highly ordered structures visible
to the naked eye. The materials are an intricate 3-D composite of
air and plastic that manipulate light in the same way that allows
opals to produce such striking colors. That's because the
materials exhibit "spatial periodicity," a sought-after
characteristic that refers to their high degree of order and
their optical properties.

The structures themselves don't appear extraordinary;
they're simply thin films that coat glass slides. But they do
hint at their optical qualities, reflecting the colors of the
rainbow like a hologram on a credit card. Under a microscope, the
detail and order of the materials is striking: Beneath the sheen,
hollow plastic spheres pack together to form a three-dimensional
structure that looks like a honeycomb. (Jenekhe points out that
there are other optical devices actually called honeycombs, but
those 2-D devices are quite different from these 3-D
structures).

"This is the next logical step in self-assembly," says
Jenekhe, the lead investigator and professor of chemical
engineering, chemistry, and materials science. "We start with
polymer molecules in solution. They self-organize into hollow
spheres, and billions and billions of them come together in a
precise, ordered way to form a larger periodic structure."

In the Science paper the scientists describe how
hollow spheres assembled together to form photonic crystals _
structures that can create and manipulate light signals
precisely, transmitting certain wavelengths while blocking
others. Photonic crystals were first envisioned by Eli
Yablonovitch, now at the University of California at Los Angeles,
more than a decade ago. In these crystals, molecules are ordered
so that light traveling through them is modulated in a highly
controlled fashion. Groups at Sandia National Laboratory, at
Allied Signal, and in the Netherlands have built photonic
crystals, but most current efforts involve a great deal of
technological hand-holding: either laborious and expensive
fabrication like drilling tiny holes into a material, or
providing a template to begin the assembly process, then somehow
removing the starting material.

The Rochester team believes its is the first photonic
crystal that literally grows itself. In the process, known as
hierarchical self-assembly, hollow spheres stack themselves into
larger structures, like bricks forming their own wall. The
device, a porous mix of a plastic framework and air pockets,
doesn't require any sort of template and isn't so much fabricated
as grown. Like opals, gems in which air and silica pack together
to produce dazzling sharp colors as light travels through them,
the alternating air spheres and plastic framework built into the
new materials manipulate the light in predictable, precise ways
as it passes through.

The team's devices are typically about a square centimeter
and 30 millionths of a meter thick, less than the thickness of a
human hair. By using chemical techniques to vary the size of the
spheres, the width of the framework, and the structure of the
shapes they create, the team is able to precisely manipulate how
light travels through the crystals - some of the same features
that determine whether opals are red, blue, or another color. In
current designs of the new materials the air pockets comprise
more than half of the structure.

"The work is extremely creative; it bodes a future world in
which we'll be able to make 3-D nanostructures that will assemble
themselves," says Yablonovitch, professor of electrical
engineering at UCLA.

Applications are widespread for a device that selectively
filters out certain wavelengths, or colors, of light. Optical
data storage and telecommunications rely on transmission and
detection of specific wavelengths, and holographic memory systems
are expected to do the same one day. The plastics might make
possible better light-emitting diodes (LEDs), materials that are
increasingly being used to produce more efficient lighting
systems. Also possible are special paints that change colors
under different light conditions _ perhaps lighter in the harsh
glare of sunlight and darker under incandescent light. Another
potential application: a super-efficient, plastic laser that
could produce intense light with a fraction of the energy now
required.

Currently, though, many scientists are simply trying to
build components that bend and steer light as handily as current
computer chips manipulate electronic signals. Today's computers
rely on semiconductors, or "electronic crystals," that give
scientists the power to control millions of electronic signals
gliding around a computer chip simultaneously. Such control
remains a dream when it comes to light: It's impossible to fit
millions of tiny conventional lenses and mirrors on a chip the
size of one's fingernail. So scientists are still searching for
materials for optical systems that would allow them to channel,
switch and manipulate optical signals with ease. The payoff would
be enormous: Light can carry thousands of times more information
than electrons and, if used instead of electronic signals, could
boost the speed of telecommunications equipment, modems and other
devices dramatically.

"What we're able to do now with electrons led to the
microelectronics revolution. We'd like to do the same with
photons. For that you need materials like these photonic crystals
that allow you to trap light and control the way it propagates,"
says Jenekhe.

The key to the work, says Jenekhe, is encoding into the
polymers information so they will organize themselves into large-
scale objects with specific characteristics. Jenekhe and graduate
student Chen worked with a polymer known as
poly(phenylquinoline)-block-polystyrene, a molecular cousin to
materials used in Styrofoam cups and in paint. By taking
advantage of the chemical properties of the material, Chen and
Jenekhe made the particles assemble into specified shapes and
sizes. Once the polymers are prepared using standard chemical
techniques, it takes them just minutes or hours to organize into
photonic crystals. Besides such crystals, Jenekhe expects self-
assembly to be useful in a variety of areas, perhaps including
biomaterials, separation media, and sensors.

"Much of nature is a product of hierarchical self-assembly,
and humans are the example par excellence. Each of us starts as a
single cell encoded with the information to guide our growth into
a larger structure _ a complete human being. Making materials
that are on their own smart, intelligent and able to orchestrate
their own growth marks the chemistry and polymer science of the
future."

Funding for the project came from the Office of Naval
Research and the National Science Foundation. The work was also
supported by an Elon Huntington Hooker graduate research
fellowship to Chen, who now works at Bell Laboratories of Lucent
Technologies.